Ring-Opening of Oxygen Heterocycles

Olah et al.724 have shown that Nafion-H induces the ring opening of oxiranes under mild conditions to afford various products. Substituted oxiranes undergo hydrolysis or alcoholysis to yield 1,2-diols or 1,2-diol monoethers, when treated with Nafion-H under mild conditions in the presence of water or alcohols, respectively. Cycloalkene oxides give the corresponding trans products stereoselectively [Eq. (5.273)]. 

Ring opening of epoxidized fatty esters with alcohols has been studied to explore the synthesis of vicinal hydroxy ethers, which are potential lubricants725 [Eq. (5.274)]. In a comparative study of ring-opening with methanol, Nafion SAC-13 showed superior activity (TOF=lmin 1 versus 0.04 min-1 after 0.5 h at 60°C) when compared to Amberlist 15 (TOF = 0.04 min-1) both exhibiting equally high selectivity ( 98%). 

Isomerization of substituted styrene oxides allows the synthesis of aldehydes in high yields726 [Eq. (5.275)]. Cycloalkene oxides do not react under these conditions, whereas 2,2,3-trimethyloxirane gives isopropyl methyl ketone (85% yield). Isomerization of oxiranes to carbonyl compounds is mechanistically similar to the pinacol rearrangement involving either the formation of an intermediate carbocation or a concerted mechanism may also be operative. Glycidic esters are transformed to a-hydroxy-/3,y-unsaturated esters in the presence of Nafion-H727 [Eq. (5.276)]. 

Methyloxirane showed a complex pattern of transformations over various solid acids in a pulse microreactor study.728 Nafion-H was found to exhibit the highest activity (30% conversion at 90°C) to produce propanal (about 22% selectivity) and cyclic dimers (substituted 1,3-dioxacycloalkanes, 60% selectivity). 

Takeuchi and co-workers736 have reported that ionization of 3,4-dimethyl-4-homoadamantanol in Magic Acid results in ionization and rearrangement to yield the 3-ethyl-5-methyl-l-adamantyl cation 178 observed by 13C NMR spectroscopy at —30°C, which, after quenching with methanol, gives ether 179 [Eq. (5.281)]. A series of known rearrangement steps and intermediates including protoadamantyl cations can account for the observation. 

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The ability of sulfur to stabilize an adjacent anion will be discussed in Chapter 46, and it means that sulfur heterocycles are much easier to deprotonate than THF. The most important of these contains two sulfur atoms dithiane. Deprotonation of dithiane occurs in between the two heteroatoms, and you can see some chemistry that arises from this on p. 1234. For tire moment, we will just show you series of reactions that illustrate nicely both dithiane chemistry and the ring opening of oxygen heterocycles in the presence of BF3, This substituted derivative of dithiane is deprotonated by BuLi in the same way to give a nucleophilic organolithium that will... 

As reagents causing intramolecular recyclizations of 2-benzopyrylium salts, nucleophiles centered at nitrogen, oxygen, sulfur, or carbon atoms were used. As a fairly general rule, the ring opening of the heterocyclic moiety leads to an orf/io-quinonoid structure, as shown in more detail in Section III,C,4,iv (Structure 201). 

Spiro[3,l-benzoxazine-4,4 -piperidine]-2(l//)-ones (80) afford 5-chloro-1,2,3,4,4a, lOb-hexa-hydrobenzo[c][2,7]naphthyridines when treated with phosphorus oxychloride. The likely reaction pathway involves initial electrophilic attack at the carbonyl oxygen atom which promotes ring opening of the heterocycle and the formation of isocyanates (81). These undergo intramolecular [4 + 2]-cyclizations to yield quinolinones, and further reaction with excess reagent then gives the final products (82) (Scheme 17) <86CPB1901>. 

NITRATE ESTERS FROM THE RING-OPENING OF STRAINED OXYGEN HETEROCYCLES... 

Examples of 47i electrocyclization and related ring openings in oxygen-and sulfur-containing heterocycles have been widely reported. The effect of substituents on the ease of photocyclization of 1-benzoxepins (1) to cyclo-buta[b]-l-benzofurans (2) has been studied.3 1,3-Dioxepin (3) is readily converted to 2,4-dioxabicyclo[3.2.0]hept-6-ene (4) on irradiation in diethyl... 

Ring-opening chemistry is characteristic of oxygen heterocycles too, and there is no need for us to revisit epoxide opening here. 

Stereocontrolled approaches to natural C-nucleosides have been based upon Baeyer-Villiger ring opening of bridged oxygen heterocycles. Noyori et al. have successfully converted lactone (59) to pseudocytidine (60 Scheme 17). 

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